feat(cockpit): add browser session client

[paul-nechifor]

• Set up the vite + vitest toolchain. They'll be set up in CI in the following PR.
• The browser can now hold a live viewer session against the relay: find the robot, adopt its manifest, subscribe, and keep the decoded frames in a store.
• No UI yet, that commes later.
• It automatically reconnects if the relay drops (e.g. because it was restarted).

• Rate and age are measured in the robot's clock, burst-delivered backlog reads as its true rate and age instead of as fresh fast data.

[greptile-apps]

Greptile Summary

Adds a browser-side cockpit session client.

• Introduces reconnecting WebTransport control and data-plane handling.
• Adds manifest-driven subscriptions, JSON decoding, channel/status stores, and React hooks.
• Adds Deno/Vite/Vitest configuration, dependencies, protocol fixtures, and session/store/transport tests.

Confidence Score: 3/5

The reconnect state-retention defect should be fixed before merging because replacement sessions can display telemetry left by a dead producer.

A new transport run reuses the Session's non-null manifest and ChannelStore, so receiving an identical manifest bypasses reset and leaves old channel slots visible under the newly confirmed producer.

Files Needing Attention: web/cockpit/src/session/session.ts

Important Files Changed

Filename	Overview
web/cockpit/src/session/session.ts	Implements session orchestration, but reconnect runs retain channel values when the replacement manifest is identical.
web/cockpit/src/session/transport.ts	Adds WebTransport connection setup, timeout handling, capped backoff, and lifecycle callbacks.
web/cockpit/src/session/store.ts	Adds bounded channel state, source-time rate and age statistics, subscriptions, resets, and sequence rebaselining.
web/cockpit/src/session/decoders/json.ts	Adds fatal UTF-8 JSON decoding with payload and preview size limits.
web/cockpit/vite.config.ts	Configures React, shared protocol aliases, relay proxying, external workspace access, and threaded Vitest execution.
web/deno.json	Adds cockpit to the Deno workspace and scopes existing root checks and tests to relay/shared code.

Sequence Diagram

sequenceDiagram
    participant UI as Cockpit UI
    participant Session
    participant Transport as ReconnectingTransport
    participant Relay
    UI->>Session: startSession()
    Session->>Transport: start()
    Transport->>Relay: fetch /api/info and connect
    Session->>Relay: hello
    Relay-->>Session: welcome and robots
    Session->>Relay: watch robot
    Relay-->>Session: manifest
    Session->>Relay: subscribe to decodable channels
    Relay-->>Session: data frames
    Session-->>UI: publish channel snapshots
    Relay--xTransport: connection closes
    Transport->>Relay: reconnect
    Note over Session: Existing manifest and slots remain
    Relay-->>Session: identical replacement manifest
    Note over Session,UI: Equal-manifest branch preserves stale slots

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

@@            Coverage Diff             @@
##             main    #3205      +/-   ##
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- Coverage   75.02%   75.01%   -0.01%     
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  Files        1120     1120              
  Lines      107091   107091              
  Branches     9719     9719              
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- Hits        80340    80339       -1     
- Misses      23941    23944       +3     
+ Partials     2810     2808       -2     

Flag	Coverage Δ
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OS-ubuntu-latest	70.69% <ø> (-0.01%)	⬇️
Py-3.10	70.68% <ø> (-0.01%)	⬇️
Py-3.11	70.69% <ø> (-0.01%)	⬇️
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Py-3.13	70.68% <ø> (-0.01%)	⬇️
Py-3.14	70.69% <ø> (ø)
Py-3.14t	70.68% <ø> (ø)
SelfHosted-Large	29.23% <ø> (ø)
SelfHosted-Linux	35.79% <ø> (+<0.01%)	⬆️
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